feat: support v-html for SVG elements

[Justineo]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Currently for older browsers (IE9~IE11) we cannot use v-html on SVG elements because they don't support innerHTML for SVG elements. While actually we can fix this without much effort and make it “just works”. As innerHTML is defined for Element interface (which SVGElement is derived from) in the latest spec, I think it's reasonable to add this for v-html.

Other information:

Related issues: #5310 #5756

Since it's a browser-specific fix and we don't run tests for IE locally, I created a repo to demonstrate the fix: https://github.com/Justineo/v-html-svg

• With latest vue on unpkg.com, see: https://justineo.github.io/v-html-svg/test-original.html
• After the fix, see: https://justineo.github.io/v-html-svg/test-fixed.html

I've tested and it should now work in IE 9~IE 11.

[Justineo]

It seems that we are waiting for #8601 to pass CI jobs as other latest PRs.

[hiendv]

Looking for this to be merged soon.